In a plasma etching apparatus, uniformity of plasma distribution near a peripheral portion of a target substrate is improved by providing a focus ring around a lower electrode on which the target substrate is mounted. However, the focus ring is gradually etched together with the target substrate during an etching process using a plasma. If the focus ring is etched, the uniformity of the plasma distribution at the peripheral portion of the target substrate is decreased. Therefore, an etching rate is changed at the peripheral portion of the target substrate and this may result in deterioration of device characteristics.
In order to measure a consumption amount of the focus ring, a thickness of the focus ring is measured by using a Vernier Caliper or the like in the state that the inside of a processing chamber of a plasma etching apparatus exposed to an atmospheric atmosphere, whenever a predetermined number of plasma etching processes is completed, for example. However, once exposed to an atmospheric atmosphere, the inside of the processing chamber needs to return to a vacuum state in order to continue the processing, which requires a time.
When the Vernier Caliper or the like is brought into contact with an inner part of the processing chamber in order to measure the thickness of the focus ring, a by-product that has been adhered to the inner part of the processing chamber may be peeled off. The by-product may adhere as a contaminant to the target substrate when the plasma etching process is performed again. In order to protect the target substrate from the contamination, it is required to perform cleaning or measure the amount of particles after the thickness of the focus ring is measured. Therefore, in the case of measuring the thickness of the focus ring, a production line is stopped during such a process and this results in decrease of a productivity.
In order to suppress the decrease of the productivity of the production line, there is required a technique for measuring the amount of the focus ring without exposing the inside of the processing chamber to the atmospheric atmosphere. As the example of such a technique, there is known, e.g., a method for irradiating laser beam to the focus ring to measure the amount of the focus ring based on an angle of a reflection light therefrom or the like (see, e.g. Japanese Patent Application Publication Nos. 2006-173223 and 2010-34416).
However, in the method for measuring the consumption amount of the focus ring based on the angle of the reflection light or the like, it is required to provide a window for introducing laser beam into the processing chamber of the plasma etching apparatus and a window for guiding the reflection light out of the processing chamber. In order to install those windows, the plasma etching apparatus needs to be modified, which increases a manufacturing cost of the plasma etching apparatus. In addition, a degree of freedom in design of the plasma etching apparatus is decreased because it is not possible to provide a member on optical paths of the laser beam and the reflection light.